Dynamics of copolymer micelles in a homopolymer melt:: Influence of the matrix molecular weight

We have studied the dynamics of styrene-isoprene (SI) block copolymer micelles in a matrix of linear entangled polyisoprene (Pr) chains, using forced Rayleigh scattering (FRS) and dynamic mechanical analysis (DMA). The molecular weight of PI has been varied from 22.000 g/mol, where the copolymer corona should be strongly swollen by the matrix chains (wet brush), to 50.000 g/mol, where corona swelling should be neglectable (dry brush). The diffusion coefficients determined by FRS showed a slowing down with increasing copolymer concentration, which is more pronounced in the case of the wet brush system. Comparing the diffusion mobility with hard-sphere colloids as a reference, a volume swelling ratio as a function of copolymer concentration and matrix molecular weight could be estimated. Surprisingly, the FRS experiments yield unusual decay-grow-decay signals for samples with very high copolymer amount. For these samples, which in analogy to colloidal hard spheres are close to the crystallization transition regime, the photobleaching process may cause a structural change, leading to complementary grating effects in the FRS signals. Further, DMA showed that the terminal relaxation process previously identified as micellar diffusion disappears for these highly concentrated samples, and an elastic plateau is obtained.